Spine showing on a BP is bad. Begin able to easily feel ribs is bad. A gushy body tone is bad. Now that I think about it I am having a hard time describing in words what a healthy BP looks like. After awhile you get the correct proportions in your head and you can just tell.
Maybe someone has already done it, but it should be possible to do a length to widest part of the body ratio. If no one else chimes in, i think I may start measuring some of my animals. If enough of us do it we should come back with some sort of range. It is a bad time of year though for this because many animals are just coming off of fasts and many have laid eggs.
There would have to be two ratios though, one for boys and another for girls. They are put together differently.